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Thank you for your question. My name is***** I have been a small animal veterinarian for 13 year.
When one pupil is smaller than then other there are two possibilities... first, that one eye is painful. The pain will cause the pupil to constrict on that side. Usually that eye is also squinting, possibly red, and just looks like it hurts. The other possibility is that there a neurologic cause. There is a syndrome that causes the eyelid to droop, the pupil to constrict, and the eye to skin in a little... this is a nerve problem called hornet's syndrome and it usually resolves without any treatment, but there are other more serious causes of nervous system related anisocoria (uneven pupil).
In general, I recommend any eye problems to be seen by a veterinarian ASAP because the eyes and vision are so critical. If you can see a vet as soon as possible that would be best. I would particularly try to find an emergency veterinarian if you are seeing redness, eye swelling, vision loss, or pain.
I hope that that helps, but if you have additional questions please let me know. If not, please rate my answer.
That is possible... usually a scratch will cause the effected eye to squint and can cause the pupil to constrict. If the cornea is scratched then it does need antibiotic drops. I hope that is the answer... scratches usually heal fast.